1. Field of the Invention
The present invention relates to a stencil mask for use in semiconductor manufacturing process, a semiconductor device, and a method of manufacturing the semiconductor device using the stencil mask.
2. Description of the Related Art
In a conventional method of manufacturing a semiconductor device, in a process of fabricating MOSFETs (metal oxide semiconductor field effect transistor) of different channel types or MOSFETs of different threshold voltages in a semiconductor substrate, a stencil mask having an opening is provided above a semiconductor substrate spaced apart at a certain distance when impurity ions are implanted into a well region, a channel region, etc.
Such a stencil mask is used in other cases, for example, when particles (charged particles such as electrons or ions, neutral particles such as atoms, molecules, neutrons, etc.) or electromagnetic waves (optical light, X-ray, etc.) are implanted into a substrate.
A stencil mask for use in a semiconductor manufacturing method is generally formed from an SOI (silicon on insulator) substrate through manufacturing steps shown in FIGS. 10 to 13.
A method of manufacturing a stencil mask will be explained below by referring to FIGS. 10 to 13.
FIG. 10 shows an ordinary SOI substrate 100. The SOI substrate 100 is provided by implanting oxygen into a silicon substrate 101 and then annealed at a high temperature to thereby form a silicon oxide film 102 having a depth of tens to hundreds of nm from the surface of the silicon substrate 101. After that, a thin silicon film 103 is formed on the silicon oxide film 102.
Next, as shown in FIG. 11, resist (not shown) is coated on the top surface of the thin silicon film 103, and a resist pattern (not shown) is formed by lithography. Then, the thin silicon film 103 is anisotropically etched by using the resist pattern as a mask to form an opening 104 in the thin silicon film 103. After forming the opening 104 in the thin silicon film 103, the resist pattern is no longer necessary and is removed.
As shown in FIG. 12, resist (not shown) is coated on the back surface of the silicon substrate 101, and a resist pattern (not shown) is formed by lithography. Subsequently, the silicon substrate 101 is treated in a chemical solution of KOH or the like, so that a portion of the silicon substrate 101 the resist coated on which was removed by the lithography is removed, leaving only the remaining portion of the silicon substrate 101 on which the resist pattern is provided. The silicon substrate portion thus left forms a supporting portion 101a. The resist pattern is then no longer necessary and is removed.
Subsequently, as shown in FIG. 13, by treating with a chemical solution such as hydrofluoric acid, the silicon oxide film 102 exposed by the step in FIG. 12, from the back surface thereof, the exposed portion of the silicon oxide film 102 is removed. In this way, a stencil mask 105 having the opening 104 formed therein is formed.
In a manufacturing method of a semiconductor device, the stencil mask 105 having the opening 104 is used when impurity ions are implanted into a semiconductor substrate.
As shown in FIG. 14, the stencil mask 105 is placed above a semiconductor substrate 106 so that the opening 104 of the stencil mask 105 aligns with an ion implantation region 107 of the semiconductor substrate 106.
Then, as shown in FIG. 15, impurity ions are implanted into the ion implantation region of the semiconductor substrate 106 through the opening 104 of the stencil mask 105 from above the stencil mask 105. On the other hand, impurity ions are not implanted into other region of the semiconductor substrate 106 than the implantation region thereof, since the other-region is masked with the stencil mask 105.
Generally, the stencil mask 105 is repeatedly used, and impurity ion implantation is repeatedly carried out. When the ion implantation is repeated, impurity ions blocked by the stencil mask 105 are accumulated on the stencil mask 105. Further, damages are accumulated on the stencil mask 105 due to the blocking of the impurity ions. Further, the stencil mask may be loaded and deformed by gravity, inertia of conveying and moving the stencil mask 105, and the like (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2002-203806).
The flexible strength of a thin film depends on physical properties represented by Young's modulus of the film, the thickness of the film and the area of the thin film region. Generally, the strength of the film is in proportion to third power of a thickness of the film. Thus, the strength of the stencil mask 105 can be high by increasing the film thickness of the stencil mask 105, so that the stencil mask 105 is prevented from being deformed.
On the other hand, as described, the opening 104 of the stencil mask 105 is formed by carrying out an anisotropic etching of an SOI substrate. Accordingly, the forming process of the opening of the stencil mask 105 depends on the material and thickness of the film to be processed. Generally, the forming process of an opening depends on a ratio (i.e., aspect ratio) of the size of the opening to be formed and the depth of the opening. It is difficult to finely process a film when the film is thick, whereas it is possible to finely process a film when the film is thin. Thus, if the film thickness of the stencil mask 105 is increased in order to enhance the strength, it becomes difficult to make a fine processing of the opening 104.
In a manufacturing method of a semiconductor device, a stencil mask is used in a step of an oblique ion implantation or a lens projection reduction aligning. In this case, ion particles are applied obliquely to the stencil mask disposed in parallel to a semiconductor substrate.
When the stencil mask 105 processed vertically to the surface thereof by anisotropic etching, as shown in FIG. 16, is used, there arises a problem that part or all of the particles passing through the opening 104 are blocked by the side wall of the opening of the stencil mask 105 and are not implanted into the semiconductor substrate 106. This problem is called shadowing.
Disadvantages of the shadowing will be explained by referring to FIG. 16. Assume that charged particles are applied at an angle θ to the stencil mask which is set above the substrate and defines the particle implantation region of the substrate. Also, assuming that the film thickness of the stencil mask is T, the width of the opening of the stencil mask and the width of the particle implantation region of the substrate in which particle are to be implanted is S1, and the width of a region of the substrate in which the particles are actually implanted is S2. Then, the relation of S2=S1−T·tan θ is established. The width S2 is smaller than the width S1 by a part corresponding to T·tan θ. T·tan θ becomes large as the angle θ becomes larger. Also, T·tan θ becomes large as the film thickness of the stencil mask larger. In other words, the rate of blocking of the particles due to shadowing is increased, as the film thickness of the stencil mask is more increased to enhance the strength. Accordingly, the particle implantation region becomes small.